Imaging the future of Arctic plant life

If you know where to look in the Arctic, you’ll find strange hexagons dotting the tundra beneath the enduring summer sun. Strange, scattered honeycomb chambers. The open-top hexagonal units shelter 1 or 2 square meters’ worth of tundra plants, passively raising the temperature within their fiberglass walls by 1-3°Celcius.

Every spring at diverse circumpolar sites, researchers deploy the six sided open-top chambers (OTCs) which act like small greenhouses. Their efforts are part of the International Tundra Experiment (ITEX), which catalogs data gathered by scientists from across the world in an effort to forecast how the Arctic tundra ecosystem will respond to global warming. Every country that lies within the Arctic Circle has participated in ITEX.

At OTC plots, and at non-enclosed control plots, scientists record careful measurements. What plants are growing, thriving or declining? When does the timing of key biological events like leaf bud burst, flowering and reproduction occur (phenology)? Does plant behavior change in response to experimental warming: heightened temperatures and longer seasons? Which species are likely to thrive in our planet’s warmer future, and which will suffer?

Because the data gathered at networked ITEX sites follows standardized measurement procedures, researchers can utilize the data to perform meta-analyses, and gain a broader image of what to expect in the Arctic’s future. The experiments can also provide data about nutrient cycling and carbon balance in the tundra ecosystem.

ITEX works to understand ecosystem response and vulnerability to change by monitoring plant species and communities across the Arctic with and without environmental manipulations.

Gathering data on plants is an arduous process. Researchers balance on boardwarks which keep bootprints and body weight off the experiment sites. They crouch down in order to measure, characterize and count plants inside the OTC plots or control plots. They record plant life cycles, and measure everything from soil temperature to the heights of inflorescences (flowers).

Advances in sensor technology have allowed additional approaches to supplement traditional data gathering. Steve Oberbauer, professor of biological sciences at Florida International University, visits Toolik Field Station north of the Brooks Range on the North Slope of Alaska to implement a robotic tram system. The tram traverses a 50 meter line on cables suspended above the tundra. It carries a suite of sensing machinery which combine to construct a 3D record of conditions on the tundra below.

“We have these 50 meter transects that are doing very detailed measurements of the surface properties of the vegetation. Telling us things like: temperature, how tall the vegetation is. We can see how much green biomass there is of the vegetation. We know what the albedo of the vegetation is, that is: how much sunlight is bouncing back off of the vegetation, how much is absorbed on the surface, which then would contribute to heating of the air.” ~ Steve Oberbauer

The project is part of the Arctic Observatory Network and works with plots associated with ITEX. Data gathered by the tram is coupled with existing hand-sampled grids, and compared with vegetation measurements taken by Earth-observing satellites.

What happens to plants?

An overview of ITEX results shows that shrubs and graminoids (grasses) grow well in warmed plots, while lichens and bryophytes (mosses and liverworts) decline. Shrubs, especially deciduous species, thrive at the expense of other species, which lowers diversity. These findings mesh with the observed increase of shrubs in locations elsewhere in the southern Arctic. Along with the increase in shrubs, one can expect to see higher vegetation canopy height (taller plants in general).

”Mosses and lichens tend to disappear so the overall diversity inside these things tends to get lower with warming, so: fewer species. And there’s some changes in species composition.” “The loss of the bryophytes and lichens and increases in shrub height is the dominant effect that we’ve seen.” “The graminoids respond better. Meaning they grow better and flower earlier. One of the things that has happened at some of our sites, the wet sites where we have just a few graminoid species (like carrix – graminoid means grass-like), carrix or grasses have filled up the chambers so much that all of this dead or organic matter leaves things in the Arctic. It’s cold and wet here; things don’t break down very fast. This is why the Arctic has all this stored carbon, all the standing dead gets trapped in there. And so they actually have less light.” ~Steve Oberbauer

At first the grasses grew large and tall in response to the OTC's warmth, but as dead material from the grasses built up over the years it ended up shading growing green leaves. Growth petered off, crowded and shaded too much by dead matter to flourish.

“We extended the season by digging out snow. And it turns out that plants don’t like the season early, don’t like an earlier season because there is a potential for frost after it’s warmed up and potentially can dry out because the soil is warmer and the period between the snowmelt and potential rain is longer so they could get dry, basically.”

"Yeah they are pretty well linked to a specific length of growing season. And that’s another thing that is very interesting that we found, that the plants, many of the plants that start early, you would think ‘Great, we’ll have a longer growing season. We can make more photosynsate, make more sugars, make more plant,’ but some of them, they start early, they finish early. They don’t have the ability to change their growing period.” “Like birch for example, the common shrub here, it leafs out early in response to snow removal, it’s going to stop the season earlier.” “They don’t have the built in flexibility or plasticity to adjust to that longer season. But I’m sure there are some plants that can deal with the longer growing season. And those will be the winners.” ~Steve Oberbauer

Vegetation changes vary across regions. Plants in the southern Arctic seem more likely to put extra energy more toward growing larger than toward spreading seeds, perhaps because the density of neighboring plants makes it unlikely for seeds to gain a foothold. Meanwhile, northern Arctic plants seem more likely to use extra energy to generate seeds, perhaps because those seeds could colonize otherwise uninhabited plots of land. It's a complex system.

Temperature, vegetation, and sea ice

The Arctic is getting greener and warmer, especially along the coast. Rick Thomen, National Weather Service climate science and service manager for the Alaska region, has an explanation. Lack of sea ice forces warmer temperatures on the normally frozen northern coast of Alaska.

“We have every reason to think that, compared to that long term normal, the late summer and fall on the North Slope will be considerably warmer than that long term normal because of the lack of sea ice that is almost certain to occur again this summer.”

“Historically up until about 2000, you have Arctic sea ice around the Arctic coast, especially from Barrow eastward by some time in October. Sea ice by definition is colder than open water. Now that the ice isn’t there and it is still far off shore that means that you have this nice 30 degree water, can’t be colder – water freezes at 29 degrees Fahrenheit, it can’t be colder than that. So any time you blow the wind off that water it limits how cold it can be on the North Slope there. Since historically that air was blowing off of ice, which historically could have been much colder than 29 degrees, basically it removes a way to get cold.” ~Rick Thomen

Nathan Healey operates the robotic tram, which begins to move away from the control tower. The tram carries sensors, and is suspended on wires above the tundra. / FrontierScientists footage

Mysterious moss

Oberbauer is excited about the very fine-scale measurements allowed by the instruments aboard his ITEX AON tram. “Already we have found out something very interesting that we hadn’t expected.” His team learned that their expectations about albedo (surface reflectiveness) were sometimes reversed. Normally, the high albedo of snow or sea ice bounces the Sun’s light back into space. The low albedo of dark-colored vegetated lands in contrast absorbs light, heating up the earth. Yet lowly moss can throw a wrench into the works.

“You would think something with a high albedo should be cooler and something with a low albedo should be warmer because it is absorbing more of that solar energy. But what we find at the small scale is, some places that relationship is reversed. That actually at higher albedo’s we saw higher temperatures and at low albedo’s we saw lower temperatures, which seems completely reversed.”

“Underneath the overstory of these plants, there’s a layer of mosses that is almost completely covering many areas.” “And what we think it is, is that it is areas that are dominated by moss – and moss when it is wet is not as reflective as when it is dry. When moss is wet it is evaporating a lot of water, and water as it evaporates cools (the whole principal of evaporative cooling with air conditioners that work by that process). So we found something not intuitive, completely unexpected and something that you wouldn’t pick up at the scale of a satellite.” ~Steve Oberbauer

Nutrients ultimately limit the system

All this vegetation growth sounds positive. Plants take in carbon dioxide as they perform photosynthesis, meaning that plant communities are a potential carbon sink – a system that might take carbon dioxide out of the atmosphere. Yet carbon dioxide isn’t the only thing that plants need to grow.

“I also worked on a project where we fumigated the tundra with high CO2 to see if tundra would respond to these high CO2 concentrations. Because the other side of the CO2 equation is the plants. So higher CO2 in the atmosphere increases greenhouse trapping and warming but higher CO2 – CO2 is the substrate, is the food, that plants take from the atmosphere to make food, in a sense you could think of it as a fertilizer of a plant. So the question was: Will Arctic plants do better with higher CO2? What it’s going to be in the future? And what we found was they do better a very short period of time and then they go back right to where they were because nutriments is what limits growth here.” ~Steve Oberbauer